Development of a Compact Primary Heat Exchanger for a Molten Salt Reactor

The first Molten Salt Reactor (MSR) was designed and tested at Oak Ridge National Laboratory (ORNL) in the 1960’s, but recent technological advancements now allow for new components, such as heat exchangers, to be created for the next generation of MSR’s and molten salt-cooled reactors. The primary (fuel salt-to-secondary salt) heat exchanger (PHX) design has been largely ignored up to this point; however, it is shown here that modern compact heat exchangers have the potential to make dramatic improvements over traditional shell-and-tube designs. Compact heat exchangers provide a higher effectiveness and more efficient use of material that offer a more cost-effective alternative to the massive, more expensive heat exchangers planned for the MSR. While this paper focuses on the application of compact heat exchangers on a Molten Salt Reactor, many of the analyses and results are similarly applicable to other fluid-to-fluid heat exchangers. The heat exchanger design in this study seeks to find a middle-ground between the dependable shell-and-tube design and the ultra-efficient, ultra-compact designs such as the Printed Circuit Heat Exchanger being developed today. Complex channel geometries and micro-scale dimensions in modern compact heat exchangers do not allow routine maintenance to be performed by standard procedures, so extended surfaces will be omitted and hydraulic diameters will be kept in the minichannel regime (minimum channel dimension between 200 μm and 3 mm) to allow for high-frequency eddy current inspection methods to be developed. Rather than using a “checkerboard” channel pattern, which requires complex header designs among other design challenges, row composition is homogeneous, and the borders between adjoining channels are removed to provide high aspect ratio rectangular channel cross-sections. Various plant layouts of smaller heat exchanger banks in a “modular” design are introduced, and the feasibility of casting such modules is assumed to be possible for the purposes of this research. FLUENT was used within ANSYS Workbench to find optimized heat transfer and hydrodynamic performance for straight-channel designs with two molten salts acting in pure counter-flow. Limiting the pressure drop to roughly that of ORNL’s Molten Salt Breeder Reactor’s shell-and-tube design, the compact heat exchanger design of interest in this study will lessen volume requirements, lower fuel salt volume, and decrease material usage. Compact heat exchangers have shown commercial feasibility in several industries but have yet to be assimilated into the nuclear industry. This intermediately-sized compact minichannel heat exchanger demonstrates that such a heat exchanger is viable for further testing. The original design of the MSR was an engineering marvel over 60 years ago, but several of its key components, namely the intermediate heat exchanger, must be updated in order for the MSR to reach its full potential.